Improved accuracy evaluation of the NPL-CsF2 primary frequency standard

Presented is quantitative evaluation of two leading uncertainties in the NPL-CsF2 fountain frequency standard. The distributed cavity phase shift evaluation is based on recent theoretical model where the cavity field is decomposed into a series of 2D Fourier components in azimuthal angle in the cylindrical cavity. Predictions of the model are reproduced experimentally. The microwave lensing effect is caused by dipole forces originating from radial variation of the microwave field amplitude and cavity apertures. The new evaluation of the two effects together with other recent improvements reduce the total type B uncertainty of NPL-CsF2 to 2.3 × 10
^-16
.